Method of driving discharge lamp, driving device, and projector

ABSTRACT

A method for driving a discharge lamp having a first electrode and a second electrode includes the steps of, in a situation where a tip portion of the first electrode becomes higher than a tip portion of the second electrode in temperature when power of the same amount is fed to the first and second electrodes during a steady operation, changing a difference between the absolute values of average current values for two polarities during one cycle of the AC current in accordance with a predetermined pattern, and setting an operation time ratio of the first electrode as an anode during one cycle so as to be smaller than an operation time ratio of the second electrode as an anode during one cycle.

BACKGROUND

1. Technical Field

The present invention relates to a method for driving a discharge lamphaving a pair of electrodes, a driving device, and a projector includinga light source incorporated with such a discharge lamp.

2. Related Art

The state of a discharge emission-type lamp changes in accordance withan operating time, and in particular, electrodes wear off while the lampis being lighted and the shapes thereof change as time passes. Forexample, if a plurality of protrusions are grown from the tip portionsof the electrodes, and the body portions of the electrodes wear offirregularly, the start point of arc may be moved or the arc length maybe changed. For this reason, the luminance of a light source device anda projector may be deteriorated and the lifespan of the discharge lampmay be shortened.

As a method of coping with the wear of the body portions of theelectrodes, a technology is known in which a lighting current equal toor more than a rated current value is supplied while the discharge lampis being lighted in a steady-state, thereby restoring the shapes of thetip portions of the electrodes (see Japanese Patent No. 3840054).

However, in a method of restoring the surface of each electrode, whichis worn off, by temporarily increasing a current value, the tip portionof the electrode is temporarily completely melted, and the arc lengthtemporarily rapidly increases. For this reason, illumination may bedeteriorated, and flicker and color irregularity may occur.

In a light source including an auxiliary mirror in order to efficientlyconverge light to be emitted forward, the electrode on the auxiliarymirror side is excessively deteriorated, and the pair of electrodes aredeteriorated unevenly.

SUMMARY

An advantage of some aspects of the invention is that it provides amethod for driving a discharge lamp, which is capable of suppressingdeterioration of illumination and preventing electrodes from wearing offunevenly, such a driving device, and a projector using the same.

According to an aspect of the invention, there is provided a method fordriving a discharge lamp that supplies an AC current to a discharge lamphaving a first electrode and a second electrode so as to producedischarge and to cause the discharge lamp to emit light. The methodincludes, in a situation where a tip portion of the first electrodebecomes higher than a tip portion of the second electrode in temperaturewhen power of the same amount is fed to the first and second electrodesduring a steady operation in which the AC current is supplied to thedischarge lamp, changing a difference between the absolute values ofaverage current values for two polarities during one cycle of the ACcurrent in accordance with a predetermined pattern, and setting a ratioof an operation time of the first electrode as an anode during one cycleso as to be smaller than a ratio of an operation time of the secondelectrode as an anode during one cycle. Here, the fact that the tipportion of the first electrode becomes higher than the tip portion ofthe second electrode in temperature means at least one of a case inwhich the maximum temperature during one cycle becomes a hightemperature, a case in which the average temperature during one cyclebecomes a high temperature, and a case in which the average temperatureduring an anode period becomes a high temperature.

With this driving method, during the steady operation, the differencebetween the absolute values of the current values for the two polaritiesduring one cycle of the AC current is changed in accordance with thepredetermined pattern. For this reason, even if the AC current whosepolarity alternates is basically used, a balance between a current valuewhen the first electrode serves as an anode and a current value when thesecond electrode serves as an anode can be appropriately changed.Therefore, a single large protrusion can be reliably maintained or grownwhile the tip portions of both electrodes are appropriately meltedalternately. As a result, both electrodes can be prevented from beingdeteriorated, and thus the lifespan of the light source can be extended.In addition, with this method, the ratio of the operation time of thefirst electrode as an anode during one cycle is set so as to be smallerthan the ratio of the operation time of the second electrode as an anodeduring one cycle. Therefore, the first electrode can be prevented frombeing liable to be higher than the second electrode in temperature whenpower of the same amount is fed, and as a result, the first electrodecan be prevented from being deteriorated earlier than the secondelectrode.

According to a specific aspect of the invention, in the above-describeddriving method, a period of one cycle of the predetermined pattern maycorrespond to a period of a plurality of cycles of the AC current,during one cycle of the predetermined pattern, the difference betweenthe absolute values of average current values during half cycle for thetwo polarities may increase or decrease, and the difference between theabsolute values of the average current values during half cycle for thetwo polarities may be changed by repetition of the predeterminedpattern. In this case, the thermal states of both electrodes and theperiphery thereof can be significantly slowly changed on a comparativelylong time scale. Therefore, steady convection can be avoided from beingformed in the light-emitting tube. As a result, part of both electrodescan be prevented from wearing off unevenly or an electrode material canbe prevented from being precipitated unevenly.

According to another aspect of the invention, a primary reflectingmirror may be disposed on the second electrode side to reflect a lightbeam generated by discharge between the first electrode and the secondelectrode so as to be emitted toward a region to be illuminated, and anauxiliary reflecting mirror may be disposed on the first electrode sideso as to be opposite the primary reflecting mirror to reflect a lightbeam from an inter-electrode space between the first electrode and thesecond electrode toward the inter-electrode space. A current value whenthe second electrode is operating as an anode may be changed within acorresponding anode period. In this case, flicker when the secondelectrode is operating as a cathode can be effectively suppressed, anddischarge can be stabilized. The reason why the first electrode on theauxiliary reflecting mirror side has a comparatively high temperature isconsidered that the first electrode is located closer to the auxiliaryreflecting mirror and is likely to be more exposed to radiation heatfrom the auxiliary reflecting mirror, or that cooling wind flowingaround the light-emitting tube is blocked by the auxiliary reflectingmirror, and cooling efficiency is lowered on a side of the firstelectrode covered with the auxiliary reflecting mirror, that is, in ahemisphere in which the first electrode is accommodated.

According to yet another aspect of the invention, a current value whenthe second electrode is operating as an anode may be changed such thatan absolute value is maximized at the end of a corresponding anodeperiod. In this case, flicker can be more effectively suppressed.

According to yet another aspect of the invention, the current value ofat least one polarity, at which the absolute value of the current valuebecomes smaller, of the two polarities during one cycle of the ACcurrent may be changed within a corresponding polarity period. In thiscase, flicker can be suppressed in an electrode that is operating as acathode, and the tip portion can be necessarily and sufficiently meltedin an electrode that is operating as an anode. As a result, dischargecan be stabilized and the shape of each electrode can be maintained.

According to yet another aspect of the invention, the current value ofat least one polarity, at which the absolute value of the current valuebecomes smaller, of the two polarities during one cycle of the ACcurrent may be changed such that the absolute value of the current valueis maximized at the end of a corresponding polarity period. In thiscase, flicker can be more effectively suppressed, and since the tipportion of the electrode is melted, the shape of each electrode can bemore effectively maintained.

According to another aspect of the invention, there is provided adriving device that supplies an AC current to a discharge lamp having afirst electrode and a second electrode so as to produce discharge and tocause the discharge lamp to emit light. The driving device includes acurrent driving circuit that, in a situation where a tip portion of thefirst electrode becomes higher than a tip portion of the secondelectrode in temperature when power of the same amount is fed to thefirst and second electrodes during a steady operation in which the ACcurrent is supplied to the discharge lamp, changes a difference betweenthe absolute values of average current values for two polarities duringone cycle of the AC current in accordance with a predetermined pattern,and sets a ratio of an operation time of the first electrode as an anodeduring one cycle so as to be smaller than a ratio of an operation timeof the second electrode as an anode during one cycle.

With this driving device, during the steady operation, the currentdriving circuit changes the difference between the absolute values ofthe current values for the two polarities during one cycle of the ACcurrent in accordance with the predetermined pattern. For this reason,even if the AC current whose polarity alternates is basically used, abalance between a current value when the first electrode serves as ananode and a current value when the second electrode serves as an anodecan be slowly changed. Therefore, a single large protrusion can bereliably maintained or grown while the tip portions of both electrodesare appropriately melted alternately. As a result, both electrodes canbe prevented from being deteriorated, and thus the lifespan of the lightsource can be extended. In addition, with this device, the ratio of theoperation time of the first electrode as an anode during one cycle isset so as to be smaller than the ratio of the operation time of thesecond electrode as an anode during one cycle. Therefore, the firstelectrode can be prevented from being liable to be higher than thesecond electrode in temperature when power of the same amount is fed,and as a result, the first electrode can be prevented from beingdeteriorated earlier than the second electrode.

According to another aspect of the invention, there is provided aprojector including a light source device that is driven by theabove-described driving method and emits light, a light modulationdevice that receives a light beam from the light source device, and aprojection optical system that projects an image formed by the lightmodulation device.

With this projector, the above-described light source device is used.Therefore, both electrodes of the light source device can be preventedfrom being deteriorated, or the electrodes can be prevented from beingdeteriorated unevenly. As a result, the projection luminance of theprojector can be maintained over a long period.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a sectional view illustrating a light source device accordingto an embodiment of the invention.

FIG. 2 is a block diagram showing the configuration of a light sourcedriving device incorporated into a light source device.

FIG. 3 is an enlarged sectional view illustrating a body portion of alight-emitting tube.

FIGS. 4A to 4C are enlarged views illustrating a repair process ofelectrodes by a light source driving device.

FIGS. 5A to 5C are diagrams illustrating the waveform of an AC currentto be supplied to both electrodes.

FIGS. 6A to 6C are diagrams illustrating a modification of an AC currentto be supplied to both electrodes.

FIGS. 7A to 7C are diagrams illustrating another modification of an ACcurrent to be supplied to both electrodes.

FIG. 8 is a flowchart illustrating the operation of a light sourcedriving device.

FIG. 9 is a diagram illustrating a projector incorporated with a lightsource device.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, a light source device incorporated with a driving devicefor a discharge lamp according to an embodiment of the invention will bedescribed with reference to the drawings,

FIG. 1 is a sectional view conceptually illustrating the structure of alight source device 100. In the light source device 100, a light sourceunit 10 includes a discharge emission-type light-emitting tube 1 servingas a discharge lamp, an elliptical reflector 2 serving as a primaryreflecting mirror, and a spherical auxiliary mirror 3 serving as anauxiliary reflecting mirror. A light source driving device 70 includes,an electrical circuit, serving as a driving device for a discharge lamp,which supplies an AC current to the light source unit 10 so as to causethe light source unit 10 to emit light in a desired state.

In the light source unit 10, the light-emitting tube 1 is formed of atransmissive silica glass tube having a central portion thereof swollenspherically. The light-emitting tube 1 includes a body portion 11 thatis a sealed body configured to emit light for illumination, and firstand second seal portions 13 and 14 that extend along an axis passingthrough both ends of the body portion 11.

In a discharge space 12 formed in the body portion 11, a tip portion ofa first electrode 15 made of tungsten and a tip portion of a secondelectrode 16 made of tungsten are disposed so as to be spaced at apredetermined distance from each other, and a compound containing raregas and halogen and mercury are filled. In respective seal portions 13and 14 extending from both ends of the body portion 11, metal foils 17 aand 17 b made of molybdenum are filled, respectively. The metal foils 17a and 17 b are electrically connected to base portions of the first andsecond electrodes 15 and 16 provided in the body portion 11,respectively. The seal portions 13 and 14 are sealed airtight from theoutside by a glass material. If power is supplied to the light-emittingtube 1 through lead wires 18 a and 18 b connected to the metal foils 17a and 17 b by the light source driving device 70 in the form of ACpulses, arc discharge is generated between a pair of electrodes 15 and16, and the body portion 11 emits light with high luminance.

The auxiliary mirror 3 is located close to the body portion 11 of thelight-emitting tube 1 and covers a substantially half of the bodyportion 11 on a front side in a beam emission direction on which thefirst electrode 15 is present. The auxiliary mirror 3 is a mold productmade of silica glass as a single body. The auxiliary mirror 3 includesan auxiliary reflecting portion 3 a that gets the light beam emittedfrom the body portion 11 of the light-emitting tube 1 toward the frontback to the body portion 11, and a support portion 3 b that is fixed tothe periphery of the first seal portion 13 in a state of supporting abase portion of the auxiliary reflecting portion 3 a. The supportportion 3 b has the first seal portion 13 inserted therein and holds theauxiliary reflecting portion 3 a in a state of being aligned with thebody portion 11,

The reflector 2 is disposed so as to be opposite a substantially half ofthe body portion 11 of the light-emitting tube 1 on a rear side in thebeam emission direction on which the second electrode 16 is present. Thereflector 2 is a mold product made of crystallized glass or silica glassas a single body. The reflector 2 includes a neck portion 2 a throughwhich the second seal portion 14 of the light-emitting tube 1 isinserted, and a primary reflecting portion 2 b that has an ellipticallycurved surface expanding from the neck portion 2 a. The neck portion 2 ahas the second seal portion 14 inserted therein and holds the primaryreflecting portion 2 b in a state of being aligned with the body portion11.

The light-emitting tube 1 is disposed along a system optical axis OAcorresponding to an axis of rotational symmetry or an optical axis ofthe primary reflecting portion 2 b such that an emission center Obetween the first and second electrodes 15 and 16 inside the bodyportion 11 becomes substantially identical to the position of a firstfocus F1 of the elliptically curved surface of the primary reflectingportion 2 b. When the light-emitting tube 1 is lighted, light beamsemitted from the arc around the emission center O of the body portion 11are reflected by the primary reflecting portion 2 b or reflected by theauxiliary reflecting portion 3 a and then further reflected by theprimary reflecting portion 2 b, and are formed as light beams convergedat the position of a second focus F2 of the elliptically curved surface.That is, the reflector 2 and the auxiliary mirror 3 have reflectingcurved surfaces substantially axisymmetric with respect to the systemoptical axis OA, and the pair of electrodes 15 and 16 are disposed suchthat the electrode axis, the center of the axis thereof, becomessubstantially identical to the system optical axis OA.

The light-emitting tube 1 is manufactured by a shrink seal whichsupports the first and second electrodes 15 and 16 individually fixed tothe tips of the metal foils 17 a and 17 b inside a silica glass tube,and in which the silica glass tube is heated from the periphery thereofby a burner at portions corresponding to both seal portions 13 and 14,softened, and contracted. The auxiliary mirror 3 is fixed to thelight-emitting tube 1 by injecting, filling, and solidifying aninorganic adhesive C in a state where the support portion 3 b isinserted through the first seal portion 13 of the light-emitting tube 1.The light-emitting tube 1 is fixed to the reflector 2 by injecting,filling, and solidifying an inorganic adhesive C in a state where thesecond seal portion 14 is inserted into the neck portion 2 a of thereflector 2.

FIG. 2 is a block diagram schematically showing the configuration of thelight source driving device 70 for lighting the light source unit 10shown in FIG. 1 in a desired state.

The light source driving device 70 generates an AC current for producingdischarge between a pair of electrodes 15 and 16 shown in FIG. 1, andcontrols the supply state of the AC current to both electrodes 15 and16. The light source driving device 70 includes a lighting device 70 a,a control device 70 b, and a DC/DC converter 70 c. Here, an examplewhere the light source driving device 70 uses an external power supplywill be described. That is, the light source driving device 70 isconnected to an AC/DC converter 81, and the AC/DC converter 81 isconnected to a commercial power supply 90 The AC/DC converter 81converts an AC voltage, which is supplied from the commercial powersupply 90, into a DC voltage.

The lighting device 70 a is a circuit that lights the light source unit10 shown in FIG. 1. The lighting device 70 a adjusts a driving waveformoutput from the light source driving device 70. Here, the drivingwaveform has factors, such as frequency of output current or voltage,amplitude, duty ratio, ratio of positive and negative amplitudes,waveform characteristics, and the like. A driving current having anarbitrary waveform characteristic, such as a square wave, a superimposedwave obtained by superimposing a triangular wave on a square wave, orthe like is output from the lighting device 70 a to the light sourceunit 10.

The control device 70 b is a circuit unit that includes a microcomputer,a memory, a sensor, an interface, and the like, and is driven by anappropriate driving voltage generated by the DC/DC converter 70 c. Thecontrol device 70 b includes a driving control section 74 that controlsthe operation state of the lighting device 70 a, a determination section75 that determinates the state of the light-emitting tube 1, and a datastorage section 76 that stores various kinds of information regardingthe operation mode of the lighting device 70 a, that is, power feedconditions and the like.

The driving control section 74 operates in accordance with a programstored in the data storage section 76 or the like. The driving controlsection 74 selects one of a power feed condition for an initialoperation and a power feed condition for a steady operation stored inthe data storage section 76 in accordance with the current state of thelight-emitting tube 1, and causes the lighting device 70 a to executethe initial operation or the steady operation in accordance with theselected power feed condition. The driving control section 74 functionsas a current driving circuit that feeds power to the light-emitting tube1 to execute a necessary lighting operation, together with the lightingdevice 70 a. In this embodiment, an operation to supply steady energy tothe second electrode 16 and the first electrode 15 is called the steadyoperation, and an operation, different from the steady operation whenlighting starts before the steady operation is executed, to supplyenergy to the second electrode 16 and the first electrode 15 is calledthe initial operation.

The determination section 75 determines a deterioration level of thelight-emitting tube 1 on the basis of the state of the light-emittingtube 1, that is a cumulative lighting time of the light-emitting tube 1,an inter-electrode voltage to the light-emitting tube 1, and the like.

The data storage section 76 stores a plurality of power feed conditionsfor the initial operation as the initial operation modes of thelight-emitting tube 1 and a plurality of power feed conditions for thesteady operation as the steady operation modes of the light-emittingtube 1, in addition to the program for the operation of the drivingcontrol section 74. Specifically, the data storage section 76 storesvarious parameters, such as set values of a current value, a frequency,and the like during the initial operation, for example, when initiationor during a rising time period. The data storage section 76 also storesvarious parameters regarding a lighting frequency, a current value, acurrent displacement, a division period, a current displacement cycle, aduty ratio, a triangular wave jump rate, and the like during the steadyoperation. Here, the current displacement means a rectangular wave-likeAC component having the same positive and negative amplitudes, that is,a DC component that should be superimposed on an AC component having alighting frequency for lighting the light-emitting tube 1. Such acurrent displacement slowly changes a difference (a relative differenceΔI which will be described below) between the absolute value of acurrent when the first electrode 15 serves as an anode and the absolutevalue of a current value when the second electrode 16 serves as an anodein accordance with a predetermined pattern. The duty ratio means a ratioR1 of an operation time of the first electrode 15 as an anode during onecycle. In this case, a ratio R2 of an operation time of the secondelectrode 16 as an anode during one cycle is a value obtained bysubtracting R1 from 1.

FIG. 3 is an enlarged sectional view illustrating the inside of the bodyportion 11 of the light-emitting tube 1 shown in FIG. 1. FIGS. 4A to 4Care conceptual views illustrating a repair process of both electrodes 15and 16. As shown in the drawings, the first and second electrodes 15 and16 in the body portion 11 individually includes tip portions 15 a and 16a, large diameter portions 15 b and 16 b, and shaft portions 15 c and 16c.

In the case of the first electrode 15 shown in FIG. 4A, a plurality ofminute concavo-convexes 63 are formed in a surface on a tip side of thetip portion 15 a. In this case, a phenomenon that a discharge startpoint is moved between the tip portion 15 a and the concavo-convexes 63,that is, flicker or arc jump occurs. Here, flicker means a phenomenonthat the discharge start point is continuously moved, and arc jump meansa phenomenon that the discharge start point is completed moved from anoriginal discharge start point. Flicker or arc jump causes displayflicker or deterioration of illuminance.

It is assumed that in order to prevent flicker or arc jump after orbefore the fact, during the steady operation of the light-emitting tube1, as described below in detail, the first and second electrodes 15 and16 are alternately subject to a cyclic repair process.

In an example shown in FIG. 4B, the current value when the firstelectrode 15 operates as an anode is set so as to be larger than thecurrent value when the second electrode 16 operates as an anode. Whenthis happens, the temperature of the tip of the first electrode 15rises, and the concavo-convexes 63 of FIGS. 4A are melted, therebyforming a molten portion 64. Let a difference between the absolute valueI1 of the current value when the first electrode 15 serves as an anodeand the absolute value I2 of the current value when the second electrode16 serves as an anode during one cycle be the relative difference ΔI,the current in the first electrode 15 is shifted by half of the relativedifference ΔI with respect to the steady state, that is, ΔI/2 as awhole. For this reason, by adjusting the increase ΔI/2 (hereinafter,referred to as displacement) due to the shift and an increase time t,the increase in temperature of the tip of the first electrode 15 can beappropriately controlled. The adjustment of the increase in temperatureof the tip of the first electrode 15 ensures that the molten portion 64can be formed in the surface of the tip portion 15 a while the tipportion 15 a substantially slightly remains, and the concavo-convexes 63can become flat. As described above, after the tip of the firstelectrode 15 is sufficiently heated, as shown in FIG. 4C, during onecycle, the absolute value of the current when the second electrode 16operates as an anode is set so as to be larger than the absolute valueof the current value when the first electrode 15 operates as an anode.When this happens, the temperature of the tip of the first electrode 15falls, and the molten portion 64 of FIG. 4B is gradually cooled. Thecooled molten portion 64 is solidified, and as shown in FIG. 4C, the tipportion 15 a is maintained in a taper shape having an adequately largesize.

In the foregoing description, repair driving has been described focusingon the first electrode 15, but for the second electrode 16, the samerepair driving may be performed simultaneously. That is, in the stateshown in FIG. 4C, the temperature of the tip of the second electrode 16rises, and the molten portion 64 is formed. Next, if the state of FIG.4C returns to the state of FIG. 4B, the temperature of the tip of thesecond electrode 16 falls, the cooled molten portion 64 is solidified.Thus, the shape of the tip portion 36 a is maintained in a taper shape.

That is, by alternately repeating the state of FIG. 4B where the firstelectrode 15 serving as an anode is heated and the second electrode 16is cooled, and the state of FIG. 4C where the second electrode 16serving as an anode is heated and the first electrode 15 is cooled, thefirst and second electrodes 15 and 16 are alternately repaired.Therefore, both electrodes 15 and 16 can be prevented from beingdeteriorated, and the lifespan of the light-emitting tube 1 can beextended.

Returning to FIG. 3, during the steady operation in which thelight-emitting tube 1 is in rated operation, an arc AR is formed in aninter-electrode space between the tip portions 15 a and 16 a of a pairof electrodes 15 and 16 by arc discharge. The arc AR and the peripherythereof become high in temperature. For this reason, convection AF fromthe arc AR upward is formed in the discharge space 12. The convection AFhits a top portion 11 a of the body portion 11, is moved along an upperhalf portion lib thereof, passes through the shaft portions 15 c and 16c of both electrodes 15 and 16, and is settled while being cooled. Thesettled convection AF is further settled along a lower half portion 11 cof the body portion 11, collides against each other below the arc AR,and rises so as to return to the arc AR upward. That is, the convectionAF is formed and circulated around both electrodes 15 and 16, but theconvection AF may include an electrode material melted and evaporated bythe arc AR. The electrode material may be topically accumulated orsegregated in the shaft portions 15 c and 16 c by steady convection andmay be grown in the form of needle, and unintended discharge may beproduced toward the upper half portion 11 b. Unintended discharge causesdeterioration of an inner wall of the body portion 11 and the lifespanof the light-emitting tube 1. In addition, when lighting by a singledriving waveform continues for a long time, the electrodes continuouslyhave a predetermined temperature distribution for a long time, and as aresult, asymmetry of the electrodes caused by a time-variant statechange tends to increase as time passes. For this reason, with respectto the AC current supplied between the first and second electrodes 15and 16, the relative difference ΔI, which corresponds to the differencebetween the absolute value I1 of the current value when the firstelectrode 15 serves as an anode and the absolute value I2 of the currentvalue when the second electrode 16 serves as an anode, is slowly andcyclically changed in a comparatively long cycle, and the temperaturedistribution of the electrodes 15 and 16 is cyclically changed. Then,the electrodes are prevented from being deteriorated unevenly, atemporal change occurs in the convection due to a difference intemperature, several hundred K, between the left and right electrodes 15and 16, and steady convection AF is prevented from being formed in thedischarge space 12. Specifically, in a cycle sufficiently larger thanthe cycle of a current waveform having a predetermined frequency(lighting frequency) to be supplied to a pair of electrodes 15 and 16, adifference (relative difference) ΔI between the absolute values ofcurrent values for the two polarities in the corresponding currentwaveform is cyclically changed. For this reason, a superimposed currentgenerated by superimposing a DC component (displacement ΔI/2) cyclicallychanging in a comparatively long cycle on an AC component of a lightingfrequency having the same positive and negative bandwidths is suppliedbetween both electrodes 15 and 16 such that the relative difference ΔIis realized. In this case, as the pattern for cyclically changing thedisplacement ΔI/2 of the current waveform, for example, a pattern inwhich the displacement cycle of the displacement ΔI/2 is divided into aplurality of division periods, and in each division period, thedisplacement ΔI/2 is maintained during a predetermined period or more isused. That is, the displacement ΔI/2 of the current waveform which issupplied to both electrodes 15 and 16 changes in a stepwise manner andcyclically increases or decreases. By adjusting the range of thedisplacement ΔI/2 and the displacement cycle, the first and secondelectrodes 15 and 16 shown in FIGS. 4B and 4C are repairedsimultaneously.

A specific driving condition will now be described. It is assumed thatthe lighting frequency supplied to both electrodes 15 and 16 is, forexample, in a range of 60 Hz to 500 Hz. In addition, each of thedivision periods constituting the displacement cycle regarding thedisplacement ΔI/2 in the current waveform is set to be, for example, 1second or more (in a specific example, 10 seconds), and in each divisionperiod, the displacement ΔI/2 is maintained to a predetermined value,which is 50% or less of the amplitude of the AC component of a lightingfrequency. Here, if the displacement ΔI/2 is divided into, for example,10 levels (in a specific example, 12 levels), the displacement cycleregarding the displacement ΔI/2 of all the division periods becomes, forexample, 10 seconds or more (in a specific example, 120 seconds). Withsuch a displacement pattern regarding the displacement ΔI/2, a repairprocess to melt and grow the tip portions 15 a and 16 a of the first andsecond electrodes 15 and 16 can be performed, and the shapes of the tipportions 15 a and 16 a can be maintained during a long period.Therefore, the movement of the arc start point or the change in the arclength can be suppressed, and the lifespan of both electrodes 15 and 16can be extended. In addition, with such a displacement pattern, thethermal states of the first and second electrodes 15 and 16 and theperiphery thereof can be slowly changed over such a long span that theconvection AF is influenced. Therefore, steady convection AF can beavoided from being formed inside the body portion 11 of thelight-emitting tube 1. As a result, the electrode material can beprevented from being grown at unintended places of both electrodes 15and 16, and both electrodes 15 and 16 can be prevented from beingdeteriorated unevenly.

Hereinafter, the fact that the first electrode 15 on the auxiliarymirror 3 side becomes higher than the second electrode 16 on thereflector 2 side in temperature will be described. First, the firstelectrode 15 is located closer to the auxiliary mirror 3 than the secondelectrode 16, and accordingly, the first electrode 15 is liable to beexposed to radiation heat from the auxiliary mirror 3. For this reason,the first electrode 15 is liable to become higher than the secondelectrode 16 in temperature. The light source unit 10 is cooled to anadequate temperature by cooling wind from a cooling device (not shown),but cooling efficiency tends to be lowered in the hemisphere of the bodyportion 11 of the light-emitting tube 1 covered with the auxiliarymirror 3. Therefore, the first electrode 15 is liable to relativelybecome higher than the second electrode 16 in temperature. As describedabove, the first electrode 15 on the auxiliary mirror 3 side is liableto become higher than the second electrode 16 in temperature, and thusthe deterioration rate of the first electrode 15 is increased. For thisreason, for example, the duty ratio of the square wave type AC currentto be supplied to the electrodes 15 and 16 during the steady operationis set so as to be smaller than 0.5. That is, the ratio R1 of theoperation time of the first electrode 15 as an anode is set so as to besmaller than the ratio R2 of the operation time of the second electrode16 as an anode. As described above, when the relative difference ΔI orthe displacementΔI/2 which is a degree of bias of the AC currentsupplied between the first and second electrodes 15 and 16 is cyclicallychanged, if the duty ratio of the AC current to the first electrode 15is set so as to be smaller than 0.5, the rise in temperature of thefirst electrode 15 on the auxiliary mirror 3 side is suppressed.Specifically, the ratio R1 of the operation time of the first electrode15 as an anode is set to, for example, 40%, and the ration R2 of theoperation time of the second electrode 16 as an anode is set to, forexample, 60%. That is, the duty ratio of the AC current which indicatesthe ratio of the operation time of the first electrode 15 as an anodeduring one cycle becomes about 0.4. Therefore, the first electrode 15 onthe auxiliary mirror 3 side can be prevented from wearing off unevenly,while the luminance of the arc AR can be ensured. The duty ratio of theAC current is not limited to 0.4, but it may be appropriately adjustedin accordance with how the first electrode 15 is liable to have a highertemperature.

In the above description, the fact that the first electrode 15 of theauxiliary mirror 3 side has a higher temperature means that during thesteady operation, power of the same amplitude or duty ratio is fed tothe first electrode 15 and the second electrode 16, and the firstelectrode 15 becomes higher than the second electrode 16 in temperature.For example, it means a case in which the maximum temperature during onecycle of the AC current to be supplied between both electrodes 15 and 16relatively becomes high on the first electrode 15 side, a case in whichthe average temperature during one cycle relatively becomes high on thefirst electrode 15 side, or a case in which the average temperatureduring an anode period relatively becomes high on the first electrode 15side.

FIGS. 5A to 5C are graphs illustrating the AC current to be supplied toa pair of electrodes 15 and 16. In each graph, the horizontal axisrepresents a time, and the vertical axis represents a current value.FIG. 5A shows an AC component for lighting between both electrodes 15and 16. This AC component is a square wave having a predetermined cycleTa corresponding to a lighting frequency, and has the same positive andnegative amplitudes ±A0 corresponding to a current value A0. In thiscase, the ratio R1=Ta1/Ta of the operation time Ta1 of the firstelectrode 15 as an anode is set so as to be smaller than the ratioR2=Ta2/Ta of the operation time Ta2 of the second electrode 16 as ananode. For this reason, the duty ratio of the AC current which indicatesthe ratio of the operation time of the first electrode 15 as an anodeduring one cycle is smaller than 0.5. FIG. 5B shows the displacementΔI/2 corresponding to a DC component to be superimposed on an ACcomponent shown in FIG. 5A. The displacement ΔI/2 changes in a stepwisemanner and cyclically increases or decreases in a cycle Tm. The cycle Tmincludes a first half cycle H1 in which the anode current of the firstelectrode 15 on the auxiliary mirror 3 side relatively becomes large,and a second half cycle H2 in which the anode current of the secondelectrode 16 on the reflector 2 side relatively becomes large. Thedisplacement ΔI/2 as the DC component changes at 12 levels in total of 6division periods P1 to P6 in the first half cycle H1 in which the anodecurrent of the first electrode 15 relatively becomes large, and 6division periods P7 to P12 in the second half cycle H2 in which theanode current of the second electrode 16 relatively becomes large. Themaximum value DM1 (the DC current value of the division period P4) ofthe anode current in the first half cycle H1 and the maximum value DM2(the DC current value of the division period P10) of the anode currentin the second half cycle H2 are all 10% of the AC component, and areidentical. FIG. 5C shows a current waveform generated by superimposingthe displacement ΔI/2 of FIG. 5B on the AC component of FIG. 5A. The ACcurrent is supplied to both electrodes 15 and 16, and the displacementΔI/2 corresponding to the DC component slowly changes to be positive ornegative in a comparatively long cycle. In addition, the ratio R1 of theoperation time of the first electrode 15 as an anode is set so as to besmaller than the ratio R2 of the operation time of the second electrode16 as an anode. Therefore, the first and second electrodes 15 and 16 canbe prevented from wearing off unevenly while the repair process isperformed, and the relative rise in temperature of the first electrode15 on the auxiliary mirror 3 side can be suppressed.

In the case of an operation pattern including one other than thedisplacement regarding the relative difference ΔI of the currentwaveform shown in FIGS. 5A to 5C, it is not necessary to maintainconstant the lighting frequency of the current to be supplied to bothelectrodes 15 and 16 or the maximum values DM1 and DM2, and differencelighting frequencies or current values may be allocated to the divisionperiods P1, P2, P3, . . . .

In the case of an operation pattern including one other than thedisplacement regarding the relative difference ΔI shown in FIGS. 5A to5C, the set values of the lighting frequency, the current value, thecurrent displacement (specifically, the range of the displacement ΔI/2of the driving waveform), the division period, the current displacementcycle, the duty ratio, and the like can be increased or decreased on thebasis of information regarding a deterioration level obtained by thedetermination section 75, for example, how both electrodes 15 and 16wear off. For example, when both electrodes 15 and 16 wear off, thelighting frequency and the current value are temporarily increased ordecreased, thereby maintaining the shapes of the tip portions 15 a and16 a of both electrodes 15 and 16. In addition, by increasing themaximum current value, that is, the range of the displacement ΔI/2, theelectrodes which are deteriorated in a time-variant manner can bereliably melted, and the shapes of the tips can be favorably maintained.

FIGS. 6A to 6C are graphs illustrating a modification of the AC currentshown in FIGS. 5A to 5C. FIG. 6A shows an AC component for lighting bothelectrodes 15 and 16. FIG. 6B shows a displacement ΔI/2 corresponding toa DC component which is superimposed on the AC component of FIG. 6A.FIG. 6C shows a current waveform in which the displacement ΔI/2 shown inFIG. 6B is superimposed on the AC component shown in FIG. 6A. In thiscase, it is common to the original AC current in that each time thedivision periods P1, P2, P3, . . . are switched, the DC component, thatis, the displacement ΔI/2 changes in a stepwise manner, and the ratio ofthe operation time of the first electrode 15 as an anode is set so as tobe relatively small. However, unlike the case shown in FIG. 5A or 5B, ofthe positive and negative polarities per cycle of the AC current, at apolarity when the second electrode 16 on the reflector 2 side operatesas an anode, the driving waveform is changed. That is, at a time Ta2 atwhich the second electrode 16 operates as an anode, a superimposed wavegenerated by superimposing a gradually increasing triangular wave on asquare wave is supplied. The average current value of the AC componentbefore being superimposed is maintained to A0, and the peak value is setto A1. Here, let the ratio of the peak value A1 to the average currentvalue A0 be a triangular wave jump rate, the triangular wave jump rateA1/A0 increases so as to be larger than the triangular wave jump rate 1of the square wave. With the adjustment of the triangular wave jumprate, when the second electrode 16 operates as a cathode, flicker can beeffectively suppressed, and discharge can be stabilized.

FIGS. 7A to 7C are graphs illustrating another modification of the ACcurrent shown in FIGS. 5A to 5C. FIG. 7A shows an AC component forlighting both electrodes 15 and 16. FIG. 7B shows a displacement ΔI/2corresponding to a DC component which is superimposed on the ACcomponent of FIG. 7A. FIG. 7C shows a current waveform in which thedisplacement ΔI/2 of FIG. 7B is superimposed on the AC component of FIG.7A. In this case, it is common to the original AC current in that eachtime the division periods P1, P2, P3, . . . are switched, the DCcomponent, that is, the displacement ΔI/2 changes in a stepwise manner,and the ratio of the operation time of the first electrode 15 as ananode is set so as to be relatively small. Meanwhile, of the positiveand negative polarities per cycle of the AC current, at a polarity whenthe absolute value of the current value becomes small, the drivingwaveform is changed. That is, when the second electrode 16 serves as ananode in the first half cycle H1, and when the first electrode 15 servesas an anode in the second half cycle H2, a superimposed wave generatedby superimposing a gradually increasing triangular wave on a square waveis supplied. The average current value of the AC component before beingsuperimposed is maintained to A0, and the peak value is set to A1. Withthe adjustment of the triangular wave jump rate A1/A0, the tip of theanode can be sufficiently melted, and flicker in the cathode can besuppressed.

FIG. 8 is a flowchart illustrating the operation of the light sourcedriving device 70. The control device 70 b reads out adequate initialdriving data necessary for starting to light the light-emitting tube 1from a driving control table stored in the data storage section 76 (StepS11).

Next, the control device 70 b controls the lighting device 70 a on thebasis of a power feed condition for an initial operation read in StepS11, and controls the initial operation including initiation and risingof the light-emitting tube 1 (Step S12).

Next, the control device 70 b reads out adequate steady driving datanecessary for maintaining the emission state of the light-emitting tube1 from the driving control table stored in the data storage section 76(Step S13). Specifically, the set values of the lighting frequency, thecurrent value, the current displacement (specifically, the range of thedisplacement ΔI/2 of the driving waveform), the division period, thecurrent displacement cycle, the duty ratio, the triangular wave jumprate, and the like during the steady operation are read out. In thiscase, a lighting waveform, such as the lighting frequency, the currentvalue, and the like, and a driving pattern or a displacement patternincluding the current displacement (specifically, the range of thedisplacement ΔI/2 of the driving waveform), the division period, thecurrent displacement cycle, the duty ratio, and the like is selected onthe basis of information regarding a deterioration level obtained by thedetermination section 75, for example, how both electrodes 15 and 16wear off.

Next, the control device 70 b controls the steady operation of thelight-emitting tube 1 of the lighting device 70 a on the basis of apower feed condition for a steady operation read in Step S13 (Step S14).A specific operation is illustrated in FIGS. 4A to 4C, 5A to 5C, 6A to6C, and 7A to 7C.

The determination section 75 determines whether or not an interruptrequest signal for requesting the end of the lighting operation of thelight source unit 10 is input during the steady operation (Step S15).When the interrupt request signal is input, information regarding thecurrent state of the light-emitting tube 1, such as a current cumulativelighting time, a voltage being currently supplied to the light-emittingtube 1, and the like, is recorded in the data storage section 76, andthen a lighting-out operation is executed.

As will be apparent from the foregoing description, according to thelight source device 100 of this embodiment, during the steady operationin which the light-emitting tube 1 is in rated operation, the relativedifference ΔI, which corresponds to the difference between the absolutevalues of the current values, for the two polarities during one cycle ofthe AC current or the displacement ΔI/2 for realizing the relativedifference as the DC component is changed in accordance with thepredetermined pattern by the lighting device 70 a that operates underthe control of the control device 70 b. For this reason, even if the ACcurrent whose polarity alternates is basically used, the balance of themaximum current value when the first electrode 15 serves as an anode andthe maximum current value when the second electrode 16 serves as ananode can be appropriately changed. Therefore, the tip portions 15 a and16 a as a single protrusion can be reliably maintained or grown whilethe tip portions 15 a and 16 a of both electrodes 15 and 16 areappropriately melted alternately. As a result, both electrodes can beprevented from being deteriorated, and thus the lifespan of the lightsource can be extended. In addition, in the light source device 100, theratio of the operation time of the first electrode 15 as an anode duringone cycle is set so as to be smaller than the ratio of the operationtime of the second electrode 16 as an anode during one cycle. Therefore,the first electrode 15 can be prevented from becoming higher than thesecond electrode 16 in temperature due to radiation heat or the likefrom the auxiliary mirror 3, and as a result, the first electrode 15 canbe prevented from being deteriorated earlier than the second electrode.

FIGS. 9 is a conceptual view illustrating the structure of a projectorincorporated with the light source device 100 of FIG. 1. A projector 200includes a light source device 100, an illumination optical system 20, acolor separation optical system 30, a light modulation section 40, across dichroic prism 50, and a projection lens 60. The light modulationsection 40 includes three liquid crystal light valves 40 a, 40 b, and 40c having the same structure.

In the projector 200, the light source device 100 includes the lightsource unit 10 and the light source driving device 70 shown in FIG. 1.The light source device 100 generates illumination light forilluminating the light modulation section 40, that is, the liquidcrystal light valves 40 a, 40 b, and 40 c, through the illuminationoptical system 20.

The illumination optical system 20 includes a parallelizing lens 22 thatparallelizes the direction of a light beam emitted from the lightsource, first and second fly-eye lenses 23 a and 23 b that constitute anintegrator optical system for dividing light into partial light beamsand superimposing the partial light beams, a polarization conversionelement 24 that arranges the light polarization direction, asuperimposing lens 25 that superimposes light having passed through bothfly-eye lenses 23 a and 23 b, and a mirror 26 that bends the opticalpath of light. In the illumination optical system 20, the parallelizinglens 22 converts the light beam direction of illumination light emittedfrom the light source unit 10 into substantially parallel light. Thefirst and second fly-eye lenses 23 a and 23 b each include a pluralityof element lenses arranged in a matrix. The element lenses constitutingthe first fly-eye lens 23 a divide light having passed through theparallelizing lens 22 into partial light beams, and collect the partiallight beams separately. The element lenses constituting the secondfly-eye lens 23 b convert the partial light beams from the first fly-eyelens 23 a into light beams having an appropriate divergence angle. Thepolarization conversion element 24 has an array of PBS, mirror,retardation film, and the like as a set of elements, and has a functionof converting the partial light beams divided by the first fly-eye lens23 a into one-directional linear polarized light. The superimposing lens25 appropriately converges illumination light having passed through thepolarization conversion element 24 as a whole such that illuminationlight can be superimposed on regions to be illuminated of the liquidcrystal light valves 40 a, 40 b, and 40 c of a subsequent stage as lightmodulation devices for respective colors. That is, illumination lighthaving passed through both fly-eye lenses 23 a and 23 b and thesuperimposing lens 25 passes through the color separation optical system30, which will be described below in detail, and is superimposed anduniformly illuminates liquid crystal panels 41 a, 41 b, and 41 c forrespective colors provided in the light modulation section 40.

The color separation optical system 30 includes first and seconddichroic mirrors 31 a and 31 b, reflecting mirrors 32 a, 32 b, 32 c, andthree field lenses 33 a, 33 b, and 33 c. The color separation opticalsystem 30 separates illumination light from the illumination opticalsystem 20 into three color light components of red (R), green (G), andblue (B), and introduces the color light components to the liquidcrystal light valves 40 a/40 b, and 40 c of a subsequent stage,respectively. More specifically, first, the first dichroic mirror 31 atransmits the R light component of the three R, G, and B lightcomponents, and reflects the G and B light components. The seconddichroic mirror 31 b reflects the G light component of the two G and Blight components, and transmits the B light component. Next, in thecolor separation optical system 30, the R light component having passedthrough the first dichroic mirror 31 a passes through the reflectingmirror 32 a and enters the field lens 33 a for controlling the incidentangle, The G light component reflected by the first dichroic mirror 31 aand further reflected by the second dichroic mirror 31 b enters thefield lens 33 b for controlling the incident angle. The B lightcomponent having passed through the second dichroic mirror 31 b passesthrough relay lenses LL1 and LL2 and the reflecting mirrors 32 b and 32c, and enters the field lens 33 c for controlling the incident angle.

The liquid crystal light valves 40 a, 40 b, and 40 c constituting thelight modulation section 40 are non-emission-type light modulationdevices for modulating the spatial intensity distribution of incidentillumination light. The liquid crystal light valves 40 a, 40 b, and 40 cinclude three liquid crystal panels 41 a, 41 b, and 41 c thatcorrespondingly receive the color light components emitted from thecolor separation optical system 30, three first polarizing filters 42 a,42 b, and 42 c that are disposed on the incident sides of the liquidcrystal panels 41 a, 41 b, and 41 c, respectively, and three secondpolarizing filters 43 a, 43 b, and 43 c that are disposed on theemission sides of the liquid crystal panels 41 a, 41 b, and 41 c,respectively. The R light component having passed through the firstdichroic mirror 31 a enters the liquid crystal light valve 40 a throughthe field lens 33 a to illuminate the liquid crystal panel 41 a of theliquid crystal light valve 40 a. The G light component reflected by thefirst and second dichroic mirrors 31 a and 31 b enters the liquidcrystal light valve 40 b through the field lens 33 b to illuminate theliquid crystal panel 41 b of the liquid crystal light valve 40 b. The Blight component having been reflected by the first dichroic mirror 31 aand passed through the second dichroic mirror 31 b enters the liquidcrystal light valve 40 c through the field lens 33 c to illuminate theliquid crystal panel 41 c of the liquid crystal light valve 40 c. Theliquid crystal panels 41 a to 41 c modulate the spatial intensitydistribution of incident illumination light in the polarizationdirection to control the polarization states of the three color lightcomponents having entered the liquid crystal panels 41 a to 41 c foreach pixel in accordance with driving signals or image signals input aselectrical signals to the liquid crystal panels 41 a to 41 c. In thiscase, the first polarizing filters 42 a to 42 c control the polarizationdirection of illumination light entering the liquid crystal panels 41 ato 41 c, respectively. The second polarizing filters 43 a to 43 cextract modulated light having a predetermined polarization directionfrom modulated light emitted from the liquid crystal panels 41 a to 41c. In this way, the liquid crystal light valves 40 a, 40 b, and 40 cform image light for the respective colors.

The cross dichroic prism 50 synthesizes image light for the respectivecolors from the liquid crystal light valves 40 a, 40 b, and 40 c. Morespecifically, the cross dichroic prism 50 has a substantially squareshape in plan view formed by affixing four rectangular prisms, and apair of dielectric multilayer films 51 a and 51 b crossing in an X shapeare formed on the boundaries of the affixed rectangular prisms. Thefirst dielectric multilayer film 51 a reflects the R light component,and the second dielectric multilayer film 51 b reflects the B lightcomponent. The cross dichroic prism 50 reflects the R light componentfrom the liquid crystal light valve 40 a by the dielectric multilayerfilm 51 a so as to be emitted to the right with respect to the traveldirection. The cross dichroic prism 50 directs the G light componentfrom the liquid crystal light valve 40 b so as to advance straight andto be emitted through the dielectric multilayer films 51 a and 51 b. Thecross dichroic prism 50 reflects the B light component from the liquidcrystal light valve 40 c by the dielectric multilayer film 51 b so as tobe emitted to the left with respect to the travel direction. In thisway, the cross dichroic prism 50 synthesizes the X, G, and B lightcomponents to produce synthesized light as image light for forming acolor image.

The projection lens 60 is a projection optical system which enlargesimage light on a desired scale of enlargement as synthesized light fromthe cross dichroic prism 50, and projects a color image onto a screen(not shown).

According to the projector 200, a pair of electrodes 15 and 16constituting the light source device 100 can be alternately repaired,and one of the pair of electrodes 15 and 16 can be prevented from beingdeteriorated early. Therefore, the projection luminance of the projector200 can be maintained over a long period.

The invention is not limited to the foregoing embodiment, but variousmodifications may be made without departing from the scope of theinvention. For example, the following modifications may be made.

For example, the displacement patterns shown in FIGS. 5A to 5C, 6A to6C, and 7A to 7C are just examples, and an AC current to be supplied toa pair of electrodes 15 and 16 may be changed in accordance with variousdisplacement patterns. In this case, the convection AP can be preventedfrom being excessively localized inside the light-emitting tube 1 whilethe first and second electrodes 15 and 16 can be repaired. In addition,one of the pair of electrodes 15 and 16 can be prevented from beingdeteriorated early.

In the foregoing embodiment, a case in which the auxiliary mirror 3 isprovided, and accordingly the first electrode 15 becomes higher than thesecond electrode 16 in temperature has been described. Even if noauxiliary mirror 3 is provided, in an air-cooled state, a difference intemperature may occur between both electrodes 15 and 16, or if bothelectrodes 15 and 16 are different in size, a difference in temperaturemay occur between both electrodes 15 and 16. In this case, by using thewaveforms shown in FIGS. 5A to 5C, 6A to 6C, and 7A to 7C, currentdriving can be achieved with the difference in temperature between bothelectrodes 15 and 16 compensated.

In the foregoing embodiment, referring to the operation of FIGS. 6A to6C, of the positive and negative polarities per cycle of the AC current,at a polarity at which the second electrode 16 on the reflector 2 sideoperates as an anode, a superimposed wave generated by superimposing agradually increasing triangular wave on a square wave is supplied.Alternatively, in addition to a polarity at which the second electrode16 operates as an anode, at a polarity at which the first electrode 15operates as an anode, a superimposed wave with a triangular wavesuperimposed may be supplied.

In the foregoing embodiment, referring to the operation of FIGS. 7A to7C, of the positive and negative polarities per cycle of the AC current,at a polarity at which the absolute value of the current value becomessmall, a superimposed wave generated by superimposing a graduallyincreasing triangular wave on a square wave is supplied. Alternatively,in addition to a polarity at which the absolute value of the currentvalue becomes small, at a polarity at which the absolute value of thecurrent value becomes large, a superimposed wave with a triangular wavesuperimposed may be supplied.

As the lamp for the light source unit 10 of the foregoing embodiment,various kinds of lamps, such as a high-pressure mercury lamp, a metalhalide lamp, or the like, may be used.

In the projector 200 of the foregoing embodiment, in order to separatelight from the light source device 100 into a plurality of partial lightbeams, a pair of fly-eye lenses 23 a and 23 b are used, but theinvention may be applied to a projector in which no fly-eye lens, thatis, no lens array is used. In addition, the fly-eye lenses 23 a and 23 bmay be substituted with a rod integrator.

The projector 200 uses the polarization conversion element 24 thatconverts light from the light source device 100 into polarized light ina specific direction, but the invention may be applied to a projector inwhich no polarization conversion element 24 is used.

In the foregoing embodiment, an example where the invention is appliedto a transmission type projector has been described, but the inventionmay be applied to a reflection type projector. The term “transmissiontype” herein means a liquid crystal light valve including a liquidcrystal panel and the like transmits light, and the term “reflectiontype” means that a liquid crystal light valve reflects light. The lightmodulation device is not limited to a liquid crystal panel. For example,a light modulation device using a micro mirror may be used.

There are a front type projector that projects an image from theprojection surface viewing side, and a rear type projector that projectsan image from the side opposite to the projection surface viewing side.The configuration of the projector shown in FIG. 9 may be applied toboth types.

In the foregoing embodiment, only an example of the projector 200 whichuses the three liquid crystal panels 41 a to 41 c has been described,but the invention may be applied to a projector which uses a singleliquid crystal panel, a projector which uses two liquid crystal panels,or a projector which uses four or more liquid crystal panels.

In the foregoing embodiment, the color light components are modulated byusing the color separation optical system 30 and the liquid crystallight valves 40 a, 40 b, and 40 c. Alternatively, the color lightcomponents may be modulated and synthesized by using a combination of acolor wheel which is illuminated by the light source device 100 and theillumination optical system 20, and a device which includes pixels ofmicro mirrors and onto which light having passed through the color wheelis irradiated.

The entire disclosure of Japanese Patent Application No. 2008-45608,filed Feb. 27, 2008 is expressly incorporated by reference herein.

1. A method for driving a discharge lamp that supplies an AC current toa discharge lamp having a first electrode and a second electrode so asto produce discharge and to cause the discharge lamp to emit light,comprising the steps of: in a situation where a tip portion of the firstelectrode becomes higher than a tip portion of the second electrode intemperature when power of the same amount is fed to the first and secondelectrodes during a steady operation in which the AC current is suppliedto the discharge lamp, changing a difference between the absolute valuesof average current values for two polarities during one cycle of the ACcurrent in accordance with a predetermined pattern, and setting anoperation time ratio of the first electrode as an anode during one cycleso as to be smaller than an operation time ratio of the second electrodeas an anode during one cycle.
 2. The method according to Claim X,wherein a period of one cycle of the predetermined pattern correspondsto a period of a plurality of cycles of the AC current, during one cycleof the predetermined pattern, the difference between the absolute valuesof average current values during half cycle for the two polaritiesincreases or decreases, and the difference between the absolute valuesof the average current values during half cycle for the two polaritiesis changed by repetition of the predetermined pattern.
 3. The methodaccording to claim 1, wherein a primary reflecting mirror is disposed onthe second electrode side to reflect a light beam generated by dischargebetween the first electrode and the second electrode so as to be emittedtoward a region to be illuminated, and an auxiliary reflecting mirror isdisposed on the first electrode side so as to be opposite the primaryreflecting mirror to reflect a light beam from an inter-electrode spacebetween the first electrode and the second electrode toward theinter-electrode space, and a current value when the second electrode isoperating as an anode is changed within a corresponding anode period. 4.The method according to claim 3, wherein a current value when the secondelectrode is operating as an anode is changed such that an absolutevalue is maximized at the end of a corresponding anode period.
 5. Themethod according to claim 1, wherein the current value of at least onepolarity, at which the absolute value of the current value becomessmaller, of the two polarities during one cycle of the AC current ischanged within a corresponding polarity period.
 6. The method accordingto claim 5, wherein the current value of at least one polarity, at whichthe absolute value of the current value becomes smaller, of the twopolarities during one cycle of the AC current is changed such that theabsolute value of the current value is maximized at the end of acorresponding polarity period.
 7. A driving device that supplies an ACcurrent to a discharge lamp having a first electrode and a secondelectrode so as to produce discharge and to cause the discharge lamp toemit light, the driving device comprising: a current driving circuitthat, in a situation where a tip portion of the first electrode becomeshigher than a tip portion of the second electrode in temperature whenpower of the same amount is fed to the first and second electrodesduring a steady operation in which the AC current is supplied to thedischarge lamp, changes a difference between the absolute values ofaverage current values for two polarities during one cycle of the ACcurrent in accordance with a predetermined pattern, and sets anoperation time ratio of the first electrode as an anode during one cycleso as to be smaller than an operation time ratio of the second electrodeas an anode during one cycle.
 8. A projector comprising: a light sourcedevice that is driven by the driving method according to claim 1 andemits light; a light modulation device that receives a light beam fromthe light source device; and a projection optical system that projectsan image formed by the light modulation device.
 9. A projectorcomprising: a light source device that is driven by the driving methodaccording to claim 2 and emits light; a light modulation device thatreceives a light beam from the light source device; and a projectionoptical system that projects an image formed by the light modulationdevice.
 10. A projector comprising: a light source device that is drivenby the driving method according to claim 3 and emits light; a lightmodulation device that receives a light beam from the light sourcedevice; and a projection optical system that projects an image formed bythe light modulation device.
 11. A projector comprising: a light sourcedevice that is driven by the driving method according to claim 4 andemits light, a light modulation device that receives a light beam fromthe light source device; and a projection optical system that projectsan image formed by the light modulation device.
 12. A projectorcomprising: a light source device that is driven by the driving methodaccording to claim 5 and emits light; a light modulation device thatreceives a light beam from the light source device; and a projectionoptical system that projects an image formed by the light modulationdevice.
 13. A projector comprising: a light source device that is drivenby the driving method according to claim 6 and emits light; a lightmodulation device that receives a light beam from the light sourcedevice; and a projection optical system that projects an image formed bythe light modulation device.
 14. A projector comprising: a light sourcedevice that have a discharge lamp having a first electrode and a secondelectrode and emits light; a driving device according to claim 7; alight modulation device that receives a light beam from the light sourcedevice; and a projection optical system that projects an image formed bythe light modulation device.